In case of demodulation of digital processing in a receiver, a receiving signal input from an antenna is passed through a first filter. In the first filter, the receiving signal is converted to the predetermined frequency lower than a frequency of input modulation signal. After eliminating any unnecessary signals from the converted analog signal by a second filter, an A/D converter converts this analog signal to a digital signal. A digital demodulator demodulates the digital signal to regenerate the original baseband signal.
In a radio apparatus, a bandwidth of the signal is narrow in comparison with the carrier frequency. As mentioned-above, a modulation signal as the receiving signal is down-converted to a frequency of low signal bandwidth and digitized by the A/D converter. As a result, sampling frequency of the A/D converter is low, and a low-cost, low speed A/D converter is used. In this case, in order to correctly obtain the demodulation output, characteristics of low distortion and low noise are required for a mixer of the frequency converter. Furthermore, input to the mixer is the demodulation signal received through a wireless transmission path, and the input level widely varies. However, the mixer is generally comprised of a non-linear circuit. Therefore, it is difficult to realize the mixer of characteristics of low distortion over a wide range of the input level.
As a solution means of this problem, the radio (wireless) receiver shown in FIG. 1 is proposed. In FIG. 1, an RF signal (modulation signal) received by the antenna 11 is input to the A/D conversion apparatus 10 through a preamplifier 12. The A/D conversion apparatus 10 converts the RF signal to a digital signal of predetermined frequency below the frequency of the RF signal. The digital signal is demodulated by the demodulator 22.
The A/D conversion apparatus 10 comprises a subtractor 13, a loop mixer 14, a mixer 15, a reference signal generator 16, a filter 17, an A/D converter 18, a D/A converter 19, a mixer 20, and a reference signal generator 21. As a whole, this component is formed as a negative feedback loop. The mixer 15 and the reference signal generator 16 comprise a first frequency converter (down converter) to convert an output signal from the subtractor 13 to a predetermined frequency below the frequency of input signal Sin. The mixer 20 and the reference signal generator 21 comprise a second frequency converter (up converter) to convert a feedback signal Sf to the subtractor 13 to a frequency nearly equal to the frequency of the input signal Sin.
The subtractor 13 subtracts the feedback signal Sf from the input signal Sin. An output signal from the subtractor 13 is input to the mixer 15 through the loop filter 14 and multiplied with a reference signal from the reference signal generator 16 to convert it to a predetermined frequency below the frequency of the input signal Sin, i.e., a frequency of bandwidth of baseband signal or an intermediate frequency. Output from the mixer 15 (output from the first frequency converter) includes an unnecessary frequency component whose frequency is above or below the predetermined frequency. The output from the mixer 15 is input to the filter 17 to pass the predetermined frequency component by eliminating the unnecessary frequency component and converted to a digital signal by the A/D converter 18.
Output from the A/D converter 18 is supplied to the demodulator 22 and the D/A converter 19 as an output signal Sout. The D/A converter 19 converts the output signal Sout to the analog signal. Output from the D/A converter 19 is input to the mixer 20, and multiplied with the reference signal from the reference signal generator 21 to convert to a frequency nearly equal to the frequency (career frequency) of the input signal Sin. Output from the mixer 20 (output from the second frequency converter) is supplied to the subtractor 13 as the feedback signal Sf.
In FIG. 1, the A/D conversion apparatus 10 is adopted as a noise shaping type (.DELTA..SIGMA. type A/D converter), and formed as the negative feedback loop as a whole. The first frequency converter consisting of the mixer 15 and the reference signal generator 16 is included in the main signal path of the negative feedback loop, i.e., a signal path from input to output of the A/D conversion apparatus 10. Accordingly, less than perfect operation (characteristics of distortion and noise) of the mixer 15 is relieved by operation of the negative feedback loop. In a theory of the negative feedback circuit, it is proved that distortion and noise existed in the main signal path are suppressed.
On the other hand, the second frequency converter consisted of the D/A converter 19, the mixer 20 and the reference signal generator 21 is included in feedback signal path of the feedback loop. The noise and distortion generated from the D/A converter 19 and the mixer 20 affects accuracy of the feedback signal Sf. Accordingly, excellent characteristics of low distortion and low noise are required for the D/A converter 19 and the mixer 20.
As a result, in FIG. 1, a number of bits of the A/D converter 18 and the D/A converter 19 are relatively set small, and dynamic range of the input signal to the D/A converter 19 and the mixer 20 is set narrow in order to easily comprise the D/A converter and the mixer of low distortion and low noise. Especially, if the number of bits of the A/D converter 18 and the D/A converter 19 is 1, a high accurate mixer can easily be realized. However, in such composition, the number of bits of the A/D converter 18 is small and a quantized noise generated from the A/D converter 18 is large. As a result, a noise chracteristics of the A/D conversion apparatus 10 falls.